Chrome scale softening process



Patented July 25,1933

UNITED STATES it I THOMAS E. DUNN, DYER B. LAKE, AND EARL T. YOUD, OF BRIDGEPORT, CONNECTICUT,

ASSIGNORS TO THE BULLARD COMPANY, OF BRIDGEPORT, CONNECTICUT, A CORPO- RATION OF CONNECTICUT CHROME SCALE SOFTENIN G PROCESS No Drawing.

Our invention relates to the manufacture of finished objects made from such metals as stainless steel, nichrome or any of the corrosion or high-temperature resisting alloys in which chromium or chromium and nickel are important ingredients. We shall hereinafter, in the claims and specification, refer to this class of metals as stainless steel. More particularly, our invention relates to the removal of scale which forms on articles made from such metals. The following specification discloses definite uses of a broadly applicable process.

In the manufacture of metal objects from such metals as referred to above, by hot working methods, such as forging, rolling, pressing, drawing, etc., as well as by heat treating and casting, the metal becomes coated with a layer of scale. This layer of scale is often very adherent and when such is the case its removal is so difiicult that it is economical practice to scrap or to return the scaled material to the furnace rather than to perform extremely expensive cleaning operations.

Methods economical and successful have been devised which remove scale from most of the ferrous or non-ferrous metals or alloys. However, none of the methods for removing scale known prior to our invention will dis solve or detach scale from chromium alloys without pitting and excessive loss of base metal. The scale which forms on these metals contains an appreciable proportion of oxide of chromium. The particular oxide in the scale which usually forms on corrosion resisting chromium steels is in a higher state of oxidation, for example, chromic oxide (Cr O and this material is an extremely refractory substance. It is tough, hard, abrasive and cohesive and it is practically insoluble in everything except very strong acid, in which it is only slightly soluble. Chromic oxide (Cr O in the native state, such as in minerals, often will not dissolve even in aqua regia. By agua regia we mean 'a mixture made up in the propor ions of one molecule HNO to three molecules of HCl.

A network of this extremely refractory chromic oxide in the scale appears to enable the scale to resist all efforts to remove it, by

Serial No. 516,516.

erties and tenacity of the scale. Efforts to remove it by chemical methods usually result in a simple dissolution of the superficially exposed, easily soluble iron, nickel or other oxides leaving exposed a continuous surface of invulnerable chromic oxide. None of the chemical baths known and used prior to our invention has any power to attack this chromic oxide without pitting and excessive loss of base metal.

We have discovered that chromic oxide in scale attached to metal can be reduced to chromous oxide under certain novel and very useful conditions. Chromous oxide (CrO) in contrast to chromic oxide (Cr O is soft, soluble and easily removable from the surface of a metal to which it is attached. Prior to our invention we believe all efforts to soften the scale on a chrome steel article, either cheaply or at great cost, have failed, but we have discovered a reducing treatment which quickly and cheaply removes or renders the scale on chrome steels soft and easily soluble in acid pickles or easily detachable by other processes. The common reducing agents,

such as S0 CO and alcohol, which are so effective in reducing the higher oxides of iron, copper and other metals do not produce the desired effect on chromic oxide (C150 However, we have discovered reducing means which are perfectly satisfactory for the purpose.

The reducing means which we have discovered are characterized by their containing nitrogen in certain chemical combinations. The nitrogen in these combinations is in a lower state of oxidation than the nitrogen, in the nitrate radical. The actual structures of these combinations, however, are very difficult to determine, due to their chemical unstability and due to the fact that they are'at times consumed in performing their useful function at almost the very time and place of origin. These combinations for practical purposes may at times be described as nitric oxide (NO) or nitrous acid (HNO or, if 100 examiner chloride is present, as nitrosyl chloride (NOCI) but when such terms are used it is to be understood that the compositions implied by the terms may be erroneous and that they are only used to describe a related combination containing nitrogen in a proper lower state of oxidation to perform a reducing function with respect to chromic oxide (Cr O As a general term describing any nitrogen compound or radical which will reduce chromic oxide (Cr O we have devised the term minitrite.

A convenient and economical way of supplying this unstable and rather transitory minitrite is to employ a mixture of nitric acid and molasses. In mixing these two substances, the molasses acts as a reducing agent and reduces the nitric acid to a minitrite. By adjusting the concentrations properly the reaction takes place without violence and a solution containing nitric acid, minitrite, sugar and other organic substances is obtained.

If a scaled chrome steel article is immersed in a mixture such as referred to above, the minitrite at once begins to reduce chromic oxide to soft, easily soluble chromous oxide and in performing this reduction the minitrite is itself oxidized back into nitric acid or other oxidation product. By constantly replenishing the supply of molasses the alternate reduction of nitric acid to a minitrite and oxidation of the minitrite to nitric acid is maintained until no more chromic oxide (C130 remains in the scale. In practice we have found that from one to three minutes of contact is suflicient to accomplish this result.

This cyclic reaction is the cause of great economy of nitric acid because nitric acid or nitrates need be added only occasionally to replenish the losses caused by side reactions which produce gaseous oxides of nitrogen which are lost as fumes.

Purely as an hypothetical or diagrammatical example of the possible mechanism of this form of the process of our invention, the following structural equations may be written. We wish it understood that this theory of the process of our invention may be entirely erroneous and offer it only as a help to the understanding of the understanding of the process and if the theory be wrong we wish it understood that the successfulness and usefulness of our invention will still remain and the protection of Letters Patent should in no wise be affected. The top portion of the diagram labeled Oxidation of sugar repre- 'sents the structural formula of a portion of a sugar molecule. The words Fresh sugar and the arrow there attached represent a supply of other sugar available in the bath and necessary to maintain the reaction. The products of oxidation of the su ar are arbitrarily represent as CO and H although other products may be formed. The central section of the diagram labeled Nitric acid, minitrite cycle represents the alternate formation of minitrite, due to the oxidation of the sugar, and reformation of nitric acid due to the interaction of minitrite and chromic oxide. The lower section of the diagram represents the passage of chromic oxide (Cr O into chromous oxide (CrO). In this illustration the formula HNO is arbitrarily chosen to represent the minitrite. I

emu SUGAR C02 and H20 mass SUGAR 00 and H20 REDUCTION (F CHHQHC OXIDE The addition of molasses to the mixture we have shown is for the purpose of providing a mild reducing agent which will reduce some of the nitric aci to a minitrite and this result can be accomplished in a number of different ways. Molasses is a cheap source of sucrose and one which is always available. Placing sucrose in solution with mi eid probably alters the molecular structure of the sucrose and it is probably this alteration product which performs the reducing function. Glucose placed in mineral acid gives alteration products which also have reducing properties. Glucose can, therefore, be used in the place of molasses and very similar results will be obtained and the cost of this material is often such as to make it a more desirable ingredient. For example, corn syrup or corn sugar, which contains glucose, can be used in the place of molasses.

We have also discovered that water-soluble carbohydrates and hydrocarbons in general will give a similar if not exactly equivalent effect, but we have found that we prefer the aliphatic compounds, such as ketones, aldoses, glycols, acids, esters, alcohols, sugars and starches to the cyclic aromatic compounds, 1'

such as phenols. The reason for this preference lies in the fact that the aliphatic substances in addition to their reducing power possess a certain passivity or protective power, while the cyclic aromatic substances are corrosive or nonprotective. An article treated in a hath made from nitric acid and a cyclic aromatic compound will usually be roughened and pitted, while the same sort of article will remain with its original surface entirely unharmed when treated in a bath in which the organic material is aliphatic. The cause of the protective effect of the aliphatic material is obscure and We are unable to explain this feature of our discovery.

We have discovered that certain substances can be added to the mixture of nitric acid and organic matter which serve to accelerate the action and have a desirable effect in hastening the effect of our invention. These we call accelerators. Such a substance is the chloride radical and this may be added to the mixture as NaCl or or in other combinations. The (wise ofthe hastening action may be due to the formation of a more active minitrite related to such combinations as NOCl. We find that the presence of the chloride radical or its equivalent is highly desirable. Among others we have found such radicals as sulfite, acetate and phosphate radicals are also usable.

To remove the scale from a chrome steel article in one typical manner according to our invention, we immerse the scaled metal object in a bath of the following composition:

Water 2250 cubic centimeters Cone. nitric acid 750 cubic centimeters Molasses 160 cubic centimeters Sodium chloride--- 60 grams The bath is maintained at a temperature of approximately 65 degrees C. and the article to be cleaned is kept immersed for a suflicient length of time. Rarely does the duration of this treatment need to exceed may require.

The invention may also be practiced by using a solution of the following compo sition:

WVater 6 pints Sodium fluoride 3 ounces HNO concentrated 2 pints Common salt 3 ounces The solution is preferably heated to a temperature in the neighborhood of 65 de rees C. The scaled article is immersed in a loath of the above composition for approximately one to four minutes, whereupon the scale will become thoroughly softened and loosened.

When practicing this invention as disclosed directly above, the cyclic formation of minitrite and the oxidation thereof whereupon chromic oxide is reduced takes place. This is evidenced by the fact that shortly after the article is immersed in the bath visible chemical activity is suddenly initiated. This activity persists for several minutes during which time the chromic oxide is reduced and the scale softened and the activity ceases as suddenly as it started the moment that all of the chromic oxide (Ci- 0 disappears. The explanation of the mechanism which is involved might be as follows:

Nitric acid contained in the bath gains access through innumerable capillary openings in the scale of an immersed article to the underlying metal. The underlying metal reduces the nitric acid to a minitrite. The minitrite thus formed may be related to such a substance as nitrosyl chloride (N001) due to the presence in the bath of chloride and other ions. The minitrite through interaction with the chromic oxide is oxidized to a higher state. This higher oxide finds its way back to the underlying metal and is reconverted to a minitrite. \Vhen all of the chromic oxide (Cr- 0;.) is consumed the underlying metal becomes coated with a protecting film of minitrite to which the metal is relatively passive. The total mass of metal required to reduce the chromic oxide Cr O in the scale is so small that perceptible pitting and etching of the surface is entirely absent.

The action of the bath disclosed above has none of the characteristics of the action of aqua regia. This is due to the fact that the bath used in this invention contains only a small fraction of the chloride required to form aqua regia and due to the fact that the bath of this invention contains an appreciable content of alkaline ions which suppresses the formation of free chlorine. The passivity of a cleaned metal article to the action of this bath is complete evidence of the absence of aqua regia because no common metal will remain passive with respect to nacent chlorine which is formed in aqua regia.

If the treatment of the article is carefully performed, as directed in this invention, the scale adhering to the article will be so thoroughly softened that it will fall away or can be as easily removed as the scale of ordinary carbon steel.

The concentrations and temperatures herein disclosed are particular embodiments of our invention which are generally applicable but are only two examples of many forms which the invention may take. lVe have found that the proportions disclosed can be varied widely, equivalents substituted for the components suggested, and that the addition of replenishing material can be made at rather extended intervals without materially impairing the scale-softening or removing properties of the bath.

The speed with which the bath functions is directly related to the temperature. For example, scale which can be softened in three minutes by contact with a bath at 65 degrees C. would require one hour for its softening in a bath at 20 degrees C. Temperatures much above 65 degrees C. are attended with excessive reduction of the nitric acid and with uneconomical losses of gaseous oxides of nitrogen offsetting further economical gains in speed of action. The optimum temperature will vary with the cost of fixed nitrogen and the nature of the work for which our invention is employed.

\Ve have herein disclosed our invention completely and fully, as well as having described in detail particular and typical cmbodiments of it. However, we do not wish to be narrowly bound to the quantities, substances and conditions herein disclosed, but wish to be limited to the true spirit of our invention as represented by the scope of the appended claims.

\Vhat we claim as our invention is:

1. The process of softening scale on a stainless steel article which consists in immersing the article in an aqueous bath composed of nitric acid and a sugar reducing agent.

21"The process of softening oxide scale on a stainless steel article which consists in immersing the article in an aqueous bath composed of nitric acid, a chloride and a sugar.

3. The process of reducing C130 scale by contacting it with a nitric acid bath containing a sugar and a soluble compound of the group consisting of chlorides and florides.

4. The process of softening scale on a stainless steel article which consists in contacting the article with a solution containing chloride ions, nitric acid, and reduction reaction products of the acid with an organic reducing agent.

THOMAS E. DUNN. DYER B. LAKE. EARL T. YOUD. 

